1. Field of the Invention
The present invention relates to a wire-cut electrical discharge machining (EDM) machine, and particularly to a wire-cut EDM machine in which the position of the liquid level of machining fluid is adjusted at the time of automatic wire threading.
2. Description of the Related Art
In wire electrical discharge machining, when a wire electrode is broken during the machining of a workpiece, returning to a machining start point and wire threading may be required. Accordingly, there is a problem that it takes a long time to perform wire threading. Meanwhile, there is a technique in which wire threading is performed at the position of occurrence of a break without returning to a machining start point in order to reduce the time required to perform a wire-threading operation. However, in the case of a workpiece having a shape with varying plate thickness, the distance from an upper nozzle to an upper surface of the workpiece varies in accordance with the position of occurrence of a break, and therefore wire-threading conditions also vary. Accordingly, depending on the position of occurrence of a break, the wire electrode is not fed straight at the time of automatic wire threading and cannot be easily passed through a machined groove. Thus, there is a problem that a reduction of wire-threading time cannot be achieved. To solve these problems, the success rate of automatic wire threading at the position of occurrence of a break needs to be improved.
One prior art technique is a wire-cut EDM machine described in Japanese Patent Application Laid-Open No. 07-040148. This is a wire-cut EDM machine of a type in which a wire electrode runs in a horizontal direction. In this wire-cut EDM machine, the success rate of wire threading is improved by discharging machining fluid at the time of automatic wire threading so that a region in which the wire electrode passes at the time of automatic wire threading may be exposed to air.
In general, in the case where automatic wire threading is performed in a wire-cut EDM machine of a type in which a wire electrode runs in a vertical direction, if the wire electrode is exposed to air until the wire electrode is inserted into a machined groove of a workpiece, the success rate of wire threading is improved. Moreover, after the wire electrode is inserted into the machined groove of the workpiece, the success rate of wire threading is improved if the wire electrode is submerged in the machining fluid.
The prior art technique described in the section of Description of the Related Art is a technique for a wire-cut EDM machine of a type in which a wire electrode runs in a horizontal direction. If the technique is used in a wire-cut EDM machine of a type in which a wire electrode runs in a vertical direction, machining fluid is to be discharged until a lower wire guide is exposed to air. Accordingly, the workpiece is exposed to air even after the wire electrode is inserted into a machined groove of a workpiece, and the success rate of wire threading decreases. Moreover, there is another problem that it takes a long time to discharge and inject the machining fluid.
As described above, if the height of the liquid level of the machining fluid is adjusted to a level close to the workpiece upper surface, the success rate of wire threading is high both before and after the wire electrode is inserted into the machined groove of the workpiece. However, in the case of a workpiece having a shape with varying plate thickness, the height of an upper surface of the workpiece varies in accordance with the position of wire threading. Accordingly, the height of the liquid level of machining fluid needs to be changed in accordance with the plate thickness of the workpiece.